The present invention relates to a color cathode ray tube, and more particularly to a color cathode ray tube having a wide deflection angle and which is equipped with an in-line type electron gun having excellent focusing characteristics.
The color cathode ray tube used as a monitor of a television receiver and an information terminal accommodates an electron gun which emits a plurality of electron beams in one end of a vacuum envelope and has a phosphor screen (image screen) on which phosphor films of a plurality of colors are coated on an inner surface of the other end of the vacuum envelope. The color cathode ray tube is also provided with a deflection yoke on an outer portion of the vacuum envelope, which the deflection yoke performs a two-dimensional scanning of electron beams from the electron gun on the phosphor screen so as to display a given image.
Further, in many color cathode ray tubes, a shadow mask which constitutes a color selection electrode is installed close to the phosphor screen and a plurality of electron beams emitted from the electron gun are made to pass through the color selection electrode and impinge on the respective phosphor films so as to form color images.
To improve the color image formed on the phosphor screen over the whole screen area, a color cathode ray tube equipped with an electron gun as part of a system which applies high voltages other than an anode voltage to a plurality of electrodes which focus the electron beams and form a multi-stage focusing lens is known.
As such an electron gun, a so-called in-line type electron gun which emits three electron beams in parallel on a plane is most common.
FIG. 8A and FIG. 8B are schematic cross-sectional views illustrating the schematic structure of the conventional in-line type electron gun, wherein FIG. 8A is a horizontal cross-sectional view as seen from a direction perpendicular to the in-line direction and FIG. 8B is a vertical cross-sectional view as seen from the in-line direction.
This electron gun includes an electron beam generating portion which is comprised of three cathodes 1 arranged in the in-line direction, a first electrode (control electrode) 2 and a second electrode (accelerating electrode) 3 and a pre-focusing lens portion which is comprised of the first electrode 2, the second electrode 3 and a third electrode 4. Further, in the direction toward the phosphor screen from the third electrode 4, a fourth electrode 5 and a fifth electrode 6 which is divided into a first fifth electrode 61 and a second fifth electrode 62 are arranged in sequence, and the fourth electrode 5 is electrically connected with the second electrode 3 so as to have the same potential and is sandwiched between the third electrode 4 and the fifth electrode 6 to which a high voltage is applied, thus forming a first-stage focusing lens (UPF: Uni-Potential Focusing).
Further, the electron gun includes the fifth electrode 6 and a sixth electrode 7 to which the anode voltage Eb which is the optimal voltage is applied, and a second-stage focusing lens (BPF: Bi-Potential Focusing) is constituted by the fifth electrode 6 and the sixth electrode 7. That is, a main lens which is formed by combining a UPF lens and a BPF lens is called a U-B lens and is popularly used in a multistage focusing type electron gun.
Numeral 8 indicates a shield cup contiguously connected to the sixth electrode 7. Further, the fifth electrode 6 is divided into two electrodes and is comprised of the first fifth electrode 61 which is electrically connected to the third electrode 4 and the second fifth electrode 62 which faces the sixth electrode 7 so as to form the second-stage focusing lens.
Further, vertical correction plates 6V and horizontal correction plates 6H are respectively mounted on the first fifth electrode 61 and the second fifth electrode 62. That is, on the second fifth electrode 62 side of the first fifth electrode 61, there are mounted the vertical correction plates 6V which are arranged such that they sandwich three electron beams individually from the horizontal direction, while on the first fifth electrode 61 side of the second fifth electrode 62, there are mounted horizontal correction plates 6H which are arranged such that they sandwich three electron beams from the vertical direction. An electrostatic quadrupole lens is constituted by these vertical correction plates 6V and the horizontal correction plates 6H.
In the in-line type electron gun having the above-mentioned constitution, a constant focusing voltage Vfs is applied to the first fifth electrode 61, and Vfd+dVf, which is obtained by superposing a dynamic voltage dVf which is increased corresponding to a deflection amount of electron beams to an optimal focusing voltage Vfd at the center of the screen, is applied to the second fifth electrode 62.
This kind of electron gun is disclosed in Japanese Laid-open Publication 189842/1990.
In this electron gun, the electrostatic quadrupole lens constituted by the vertical correction plates 6V and the horizontal correction plates 6H are installed, Vfs is applied to the vertical correction plates 6V, Vfd+dVf is applied to the horizontal correction plates 6H, and Vfs less than Vfd+dVf is established; and, hence, the quadrupole lens acts as a focusing lens for the horizontal direction of the electron beams an acts as a divergent lens for the vertical direction.
Due to such actions, the deflection aberration which becomes a cause of a lateral defocusing phenomenon or a horizontal crush of the beam spot generated around the periphery of the screen is corrected. Further, when the electron beams are focused at the center of the screen, a curvature-of-field (curvature-of-field aberration, curvature-of-field defocusing) which causes excessive focusing is generated.
With respect to this phenomenon, since the focusing voltage of the second fifth electrode 62 which faces the sixth electrode (anode) 7 becomes high due to the relationship of Vfs less than Vfd+dVf, the focusing action of the above-mentioned second-stage focusing lens is weakened so that the curvature-of-field can be simultaneously corrected and excellent focusing characteristics can be obtained over the whole screen.
Here, corresponding to the widening of the deflection angle of the electron beams, the above-mentioned deflection aberration and the curvature-of-field are also increased so that in the color cathode ray tubes which have been popularly used as display monitors for personal computers or electronic computer terminals, the deflection angle of the electron beams has been set to approximately 90xc2x0.
In converting this deflection angle of 90xc2x0 into the shape of the color cathode ray tube, by designating the diagonal size of an effective screen as De and the distance from the center of the phosphor screen to the end portion of the focusing electrode which forms the main lens of the electronic gun and faces the anode electrode as Lg, the ratio De/Lg becomes De/Lg ≅1.4.
To consider the mounting of such a color cathode ray tube in a display monitor device, it is preferable that the total length of the color cathode ray tube is short. In case the deflection angle is widened to 100xc2x0, for example, to make the color cathode ray tube short, with the same diagonal size of effective screen De of the color cathode ray tube having a deflection angle of 90xc2x0, the distance Lg from the center of the phosphor screen to the opposing end portion of the focusing electrode, which forms the main lens of the electron gun and faces the anode electrode in an opposed manner, can be shortened approximately inversely proportional to the tangent angle (expressed as tan (Amax/2) in case the maximum deflection angle is Amax).
Further, in case this deflection angle is increased, it becomes necessary to increase the above-mentioned dynamic voltage dVf. However, in view of the limitation imposed on the actual designing of a dynamic focusing circuit at the monitor side, a sufficient dynamic voltage cannot be applied, thus giving rise to the deterioration of the focusing at the periphery of the screen.
Further, the increase of the dynamic voltage dVf makes the drive source per se expensive so that it is an important task to suppress the dynamic voltage dVf to a low level.
According to the present invention, it has been found when the relationship of Vfs greater than Vfd+dVf is set at the screen center and the astigmatism is set to be relatively large at the screen center, although the dynamic voltage dVf becomes small, the diameter of the beam spot on the screen is enlarged so that the focusing characteristics are deteriorated.
It is an object of the present invention to provide a color cathode ray tube having a total length which is short, thus providing a monitoring device which is compact and simultaneously exhibiting excellent focusing characteristics over the entire screen.
To achieve the above object, contrary to the above-mentioned electron gun used in the conventional color cathode ray tube where the relationship of Vfs less than Vfd+dVf is set at the periphery of the screen, the present invention is characterized by applying the dynamic voltage in such a manner that the relationship of Vfs greater than Vfd+dVf is set at the screen center.
The typical constitutions of the present invention are as follows.
(1) The color cathode ray tube has the relationship that the ratio between the diagonal size De of the phosphor screen (effective screen) on which at least the phosphors of three colors are coated and the distance Lg from the center of the phosphor screen to the end portion of the focusing electrode which faces the anode electrode in an opposed manner and forms the main lens portion of the electron gun is set to De/Lg greater than 1.5, and the focusing electrode and the anode electrode are constituted such that they have single opening portions whose opening size in the horizontal direction is not less than 68% of the outer diameter of a neck portion, and end surfaces of the opening portions face each other in an opposed manner in the tube axis direction along which the electron beams pass.
(2) The focusing electrode adjacent to the anode electrode which forms the main lens of the electron gun in (1) and to which the anode potential is applied is divided in the tube axis direction; in addition to a constant focusing voltage, a dynamic voltage which is changed corresponding to the deflection amount of the electron beam is superposed to one electrode of the divided focusing electrodes adjacent to the anode electrode, an electrostatic quadrupole lens which changes in the intensity thereof in response to application of the dynamic voltage is provided to at least one portion; and, further, a curvature-of-field correction lens which weakens its focusing action in the horizontal direction as well as in the vertical direction against three electron beams corresponding to an increase of the dynamic voltage is provided at the anode electrode side in the tube axis direction of the electrostatic quadrupole lens.
(3) The electrostatic quadrupole lens and the curvature-of-field correction lens are provided in the electron gun of (1), and further, an electron beam generating portion which generates three electron beams arranged in the horizontal direction is comprised of a cathode, a first electrode and a second electrode which are arranged in the anode electrode direction, and a third electrode and a fourth electrode which are arranged adjacent to the second electrode, and the third electrode is divided into two elements, thus forming another electrostatic quadrupole lens.
(4) The voltage which corrects the astigmatism generated from the potential difference between the just focusing voltages in the horizontal, direction and the vertical direction of the main lens on the electron beam spot at the center of the screen formed of the phosphor screen in (1), (2) or (3) is set to 1.4xe2x88x922.2 kV, the dynamic voltage at the corners of the screen is set to not more than 600 V, and the diameter of the electron beam spot at the center of the screen is set to not more than 0.6 mm.
(5) The outer diameter of the neck portion which accommodates the electron gun of (1), (2), (3) or (4) and constitutes the vacuum envelope is set to substantially 29 mm, while the opening portions of the focusing electrode and the anode electrode which face each other in an opposed manner are formed into a single laterally elongated shape, wherein the opening size in the horizontal direction is set to not less than 20 mm and the opening size in the vertical direction is set to not less than 9.5 mm.
In the electron gun of the conventional color cathode ray tube, the astigmatism of the main lens is set to approximately hull and the electron beams are focused on the phosphor screen without receiving the action of the electrostatic quadrupole lens at the center of the screen while the action of the electrostatic quadrupole lens is maximized at the periphery of the screen and hence, the defocusing of the beam spot generated by the deflection aberration is corrected. To the contrary, in the electron-gun used for the color cathode ray tube of the present invention, a given astigmatism is provided to the main lens and the action of the electrostatic quadrupole lens at the center of the screen is maximized so that the astigmatism of the main lens is corrected and the beam spot is made to approach a perfect circle.
Since the action of the electrostatic quadrupole lens decreases at the periphery of the screen, the defocusing of the beam spot is corrected in such a manner that the deflection aberration offsets the astigmatism of the main lens. Further, in the present invention, besides the electrostatic quadrupole lens, for correcting the curvature-of-field independently, another correction lens is provided for focusing the electron beams vertically and horizontally at the time of applying the dynamic voltage and for weakening the focusing action corresponding to the increase of the deflection amount of the electron beams and, hence, the dynamic voltage can be reduced more than in the conventional electron gun.
Further, by establishing the relationship that the ratio between the effective screen diagonal size De and the distance Lg from the center of the phosphor screen to the end portion of the focusing electrode which faces the anode electrode in an opposed manner and forms the main lens portion of the electron gun is set to De/Lg greater than 1.5 and by making the structures of respective opposing portions of the focusing electrode and the anode electrode have a large opening size with the opening size in the horizontal direction being not less than 20 mm and the opening size in the vertical direction being not less than 9.5 mm, the focusing voltage is reduced by the main beam having the large opening size, while by providing the correction lens for correcting the curvature-of-field, the diameter of the beam spot can be made small and the effect that the dynamic voltage drive is further reduced can be obtained.
FIG. 9A and FIG. 9B show waveforms of the dynamic voltages applied to the electron gun, wherein FIG. 9A is the waveform of the dynamic voltage applied to the electron gun of the conventional color cathode ray tube and FIG. 9B is the waveform of the dynamic voltage applied to the electron gun of the color cathode ray tube of the present invention. In the drawing, 1H indicates one horizontal period and 1V indicates one vertical period.
FIG. 10A and FIG. 10B are explanatory views of an optical model of the electron gun, wherein FIG. 10A indicates the optical model of the electron gun of the conventional color cathode ray tube and FIG. 10B indicates the optical model of the electron gun of the color cathode ray tube of the present invention.
In the conventional electron gun, the dynamic voltage is applied to the focusing electrode which constitutes the main lens ML (convex lens in the horizontal direction as well as in the vertical direction in FIG. 10A) so as to weaken the intensity of the main lens at the time of deflecting the electron beams toward the periphery of the screen (in FIG. 10A, this phenomenon is expressed by a lens having a thickness smaller in the horizontal direction as well as in the vertical direction than the main lens ML in case of the screen center) thus correcting the curvature-of-field aberration, and simultaneously, the electrostatic quadrupole lens QL (concave lens in the vertical direction and convex lens in the horizontal direction in FIG. 10A) corrects the astigmatism.
In general, although the intensity of the electrostatic quadrupole lens can be relatively easily adjusted, since the intensity of the main lens is set to a proper intensity so as to focus the electron beam to the phosphor screen, a sufficient adjustment of the intensity of the main lens is difficult.
Accordingly, although the correction sensitivity of the astigmatism can be enhanced by increasing the intensity of the electrostatic quadrupole lens, with respect to the curvature-of-field (curvature-of-field aberration), it is difficult to enhance the correction sensitivity of the astigmatism-more than the correction sensitivity determined by the intensity of the main lens.
It is necessary to take a balance between the correction sensitivity of the astigmatism and the correction sensitivity of the curvature-of-field aberration, and, hence, it is impossible to enhance only the correction sensitivity of the astigmatism. Accordingly, in the conventional electron gun, there is almost no space for reducing the dynamic voltage by enhancing the sensitivity.
On the other hand, the present invention is characterized by providing a curvature-of-field aberration correction lens FL (FIG. 10B shows only a convex lens in the horizontal direction in case of the screen center) independently from the main lens ML, besides the electrostatic quadrupole lens QL for correcting the astigmatism.
The curvature-of-field aberration correction lens FL is a non-axisymmetric lens formed by facing longitudinally elongated openings (vertically elongated openings) in an opposed manner, that is, a slit lens (not shown in the drawing) and the focusing force is intensified more in the horizontal direction than in the vertical direction.
At the corners of the screen, as can be also understood from the waveform of the dynamic voltage in FIG. 9B, the potential difference at the electrostatic quadrupole lens portion and the slit lens portion is small, and, hence, the intensity of the lens is weak. At the center of the screen, the lens intensity of both of the electrostatic quadrupole lens and the curvature-of-field aberration correction lens (=slit lens) is maximized, and, hence, the astigmatism of the main lens is corrected. At the corners of the screen, the intensity of the electrostatic quadrupole lens, the intensity of the curvature-of-field aberration correction lens and the intensity of the main lens are respectively minimized.
Accordingly, the focusing action against the electron beams at the corner of the screen is minimized, and, hence, the curvature-of-field aberration can be corrected. Although the effect that the curvature-of -field aberration is corrected by the dynamic voltage applied to the main lens electrode is the same as the effect of the above-mentioned conventional electron gun, the correction effect is increased by an amount of the correction effect obtained at the slit lens, and, hence, the curvature-of-field aberration can be corrected with the low dynamic voltage.
In the main lens of the electron gun used f or the color cathode ray tube of the present invention, the opposing portions of the electrodes which face each other in an opposed manner have a large opening size, wherein the opening size in the horizontal direction is set to not less than 20 mm and the opening size in the vertical direction is set to not less than 9.5 mm so that the focusing voltage Vf of the electron beam can be reduced, and, accordingly, the dynamic voltage superposed on the focusing voltage can be reduced. As well known, a main lens having a large opening reduces the spherical aberration, and, hence, the diameter of the electron beam spot can be minimized.
The present invention is not limited to the above-mentioned constitutions and embodiments described hereinafter and various modifications are possible without departing from the technical concept of the present invention.